The present invention relates to a method and apparatus for injecting NO.sub.x inhibiting liquid reagent into the flue gas of a boiler, in order to reduce the emission of NO.sub.x.
NO.sub.x emissions are a common problem encountered during the operation of boilers due to extremely high temperatures involved in boiler operation. Concern for the environment has resulted in the development of several methods and devices to combat the NO.sub.x pollutant problem.
U.S. Pat. No. 4,208,386 discloses a process for reducing NO.sub.x emissions found in combustion effluent, through the use of urea or a urea solution sprayed onto the flue gas having a temperature window of 1300.degree. F. to 2000.degree. F. It has been found that NO.sub.x control is best if the reagent is sprayed within this temperature window.
U.S. Pat. No. 4,842,834 discloses a process and apparatus for reducing the concentration of pollutants in flue gas due to combustion of the fuel. An effluent treatment fluid is injected at independently variable droplet sizes and distances into a wide variety of distribution patterns within the flue gas passage. An atomization conduit extends into the flue gas and is positioned coaxially around a treatment fluid conduit to supply an atomization fluid.
U.S. Pat. No. 4,985,218 discloses a process and apparatus for reducing NO.sub.x concentrations in a flue gas from a combustion chamber. The process and apparatus enable the injection of a flue gas treatment fluid at a low treatment fluid flow rate, yet provide an even dispersion of treatment fluid within the flue gas passage with little or no clogging. An atomization conduit, positioned coaxially within a treatment fluid supply conduit, extends into the flue gas and supplies an atomization fluid, such as steam or air. A treatment fluid is supplied through a conduit and through at least one jet in the atomization conduit wall at a velocity of between 2 to 60 feet per second, causing atomization of the treatment fluid within a nozzle. The treatment fluid used to reduce NO.sub.x emissions is preferably comprised of an aqueous solution of urea, ammonia, nitrogenated hydrocarbon, oxygenated hydrocarbon, hydrocarbon or combinations thereof.
U.S. Pat. No. 5,058,514 discloses a process for controlling acid gas emissions in flue gases. An in-furnace injection process is used to control both SO.sub.2 and NO.sub.x emission from the flue gases. An aqueous solution aimed at reducing the pollutants is injected into the furnace at a temperature range or window between 900.degree. C. to 1350.degree. C. At optimal operating conditions, about 80% of the SO.sub.2 and 90% of the NO.sub.x are removed. Similarly, urea has been found to be the preferred nitrogenous progenitor additive. The urea can be injected in a cross current, concurrent or counter current direction to the flue gas flow.
On most occasions, the ability to inject the reactant into a specified temperature window has presented several application problems. One such problem is caused due to the appropriate temperature window moving upstream gas flow-wise with a decrease in boiler load and downstream with an increase in load. Due to varying load changes within the boiler, a given flue gas temperature will move back and forth in relation to boiler load changes. Thus, varying boiler load causes a shifting of temperatures within the flue gas passageways so that injection may not take place at the appropriate flue gas temperature.